The present invention is related to load responsive brake apparatus and more particularly to a variable load valve device of the type in which the load proportioning aspect is provided by a lever arrangement for interconnecting the pistons controlling the valving between the variable load valve inlet and outlet to obtain mechanical advantage in effecting closure of the valving when the brake pressure is at a level commensurate with the vehicle load.
Vehicles employing load dependent, brake force regulation on only one axle, for example the rear axle, are subject to wheel lock-up on the front axle under certain braking conditions. During brake requirements, a partially or fully loaded vehicle has its rear axle brake pressure modulated in accordance with the rear axle load, while full brake pressure is directed to the front axle. Due to the reduced rear axle brake force, the operator may find it necessary to increase the brake pressure to properly control the vehicle, thus causing the front axle wheels to lock up. In addition, brake shoe lining wear on the front axle wheels is accelerated.
In order to counteract these disadvantages, load dependent or variable load valves of the above type have been developed to withhold brake pressure regulation during the initial phase of a relatively heavy brake application, or throughout the brake process during a relatively light brake application. These developments rely upon preadjustment of components at the factory and thus have the disadvantage of not being able to be adjusted in the field for adaptation to different conditions of vehicle utilization except by disassembly of the device.
In copending application, Ser. No. 139,616, assigned to the assignee of the present invention, there is provided an arrangement by which an external adjustment can be made to set the desired level of normal brake pressure development prior to initiation of load-dependent brake pressure regulation. Such an arrangement permits field adjustment to accommodate different conditions of use of the vehicle, but introduces the possibility of the normal brake pressure development being adjusted such as to adversely affect the range of load-dependent pressure regulation. For example, the adjustment might be such that the initial brake pressure development continues until a relatively high pressure is reached before the load-adjustment pressure phase begins. In such a case, only a relatively short range of load-dependent brake pressure regulation is possible, and depending upon the condition of use of the vehicle, may be undesirable.